For purposes of this application, the term “bridge” as used herein refers to bridges that cross natural or manmade boundaries or obstacles, such as rivers and streams, ravines, canyons or the like, structures that cross underlying roads, railroads or any other structural contrivance having a free space thereunder and which conveniently connects two points for the purpose of facilitating movement of vehicular traffic. Furthermore, the bridge may have one or more spans, and a bridge deck that provides a surface on which vehicles move. The bridge deck typically comprises a pavement surface, which may be a more-or-less continuous slab of pavement material that is usually less than about a foot thick, and a bridge deck support structure that supports the pavement. Such support structure may be of a box-girder, beam-girder or other design. While Applicants' method is disclosed as being used in conjunction with a pavement bridge deck, the invention disclosed herein should be usable with all manner of bridge decking.
Monitoring weights of vehicles crossing a bridge is useful for a number of bridge management functions related to public safety, maintenance budgetary considerations and capital infrastructure projects. In this regard, the vehicles of primary importance are loaded trucks, and especially trucks that may be loaded above allowed limits with respect to bridge design load limits and known bridge structural deficiencies. Examples of bridge management functions are developing input data for advanced planning of bridge pavement and structural support enhancements and immediate alerting or logging incidents of overweight trucks for motivating urgent pavement and structural inspection and subsequent maintenance and/or repair. Bridge load monitoring is especially important considering the large number of aging and structurally-deficient bridges that are heavily used, this importance being particularly emphasized by well publicized catastrophic collapses of bridges with their attendant loss of life.
Conventional prior art systems for weighing vehicles crossing a bridge are usually in-pavement systems. These systems typically incorporate sensors that are installed in the pavement just below the pavement surface, and which utilize bending or flexure members similar to load cells. Usually these bending members are installed in the pavement of a bridge approach, and less frequently are installed in pavement of a bridge deck because of significant difficulties encountered by such an installation on bridge spans. These systems are also expensive in both equipment and installation costs. Further, these systems experience frequent failures, especially on more heavily used bridges, because of high cycle deflection and fatigue of sensing members, thereby requiring frequent maintenance. Because these sensors are installed in the bridge approach or bridge deck pavement, this maintenance requires disruption of traffic flow over what is typically a main traffic route, and these main routes usually have inconvenient detour routes available.
An off-road weigh-in-motion system for roadways has been disclosed in Applicants' previous patent (U.S. Pat. No. 6,692,567) which is incorporated in its entirety herein by reference. As disclosed, seismic signals, generated by vehicles traveling on a roadway, propagate through a distance of surface layer of earth adjacent the roadbed and are measured by a seismic sensor. The computed energy represented in the measured sensor output signal is indicative of vehicle weight. Such a system could be deployed alongside bridge approaches, but not on a bridge deck. Furthermore, bridge approaches often have a concentration of nearby access ramps and service roads thereby making practical application of this system for bridge vehicular loading difficult or impossible to implement because of interfering seismic signals from vehicles traveling on nearby roadway approaches. For this reason, Applicants' prior art only discloses a seismic system along a single roadway, and in some embodiments uses a rumble strip to generate a specific, identifiable seismic signal.
A distributed fiber optic detection system based on Sagnac and Michelson interferometers is proposed by Udd (U.S. Pat. No. 5,636,021) for simultaneously measuring location and amplitude of an acoustic disturbance on a bridge as well as simultaneously measuring slowly-changing local and regional longitudinal strains in bridge structure. Udd proposes a system wherein fiber optic sensor loops are installed within the bridge deck and encompass the entire bridge length.
The portion of the Udd system sensor having capability for simultaneously measuring location and amplitude of acoustic disturbance on a bridge is a continuously-distributed measurement sensor. Udd proposes to accomplish this by measuring two signals: a signal that is dependent on both disturbance magnitude and disturbance location in the loop (Sagnac interferometer component) and a signal that is dependent on disturbance amplitude but is independent of disturbance location (Michelson interferometer component). Significantly, Udd's system will be of little practical value for bridge applications because commonly there will be present more than one vehicle on a bridge; this is especially the case on bridges with longer spans and/or on moderately-to-heavily utilized bridges. In this common multiple vehicle situation, a determination of disturbance magnitude will be an indication of a sum of the weights of all vehicles present on a bridge span and a corresponding disturbance location will not represent an actual vehicle location.
The aspect of the Udd sensor system proposed for measuring local and regional longitudinal strain in a bridge structure is achieved by including local spectral reflective elements at points along an optical fiber cable in bridge pavement encompassing the entire length of a bridge. However, and as noted, this local strain information for a bridge cannot in general determine individual vehicle weight because local strain on a bridge is significantly influenced by weights and locations of all vehicles on the bridge span. Consequently, this system cannot measure weights of individual vehicles because local strain in a bridge span is sensitive to weights of all vehicles in all lanes of the bridge span. This fact may have motivated Udd's selection of locations for deploying local strain elements to be at or near span joint areas above vertical bridge supports. Local strains near the pavement surface at these particular between-span locations will be among the largest experienced throughout the bridge, and Udd appropriately notes that these strain elements are useful for indicating bridge health.
From the foregoing, the Udd system may provide useful information on the strains within bridge deck structure, but it is believed that this information, as disclosed, cannot be used for weigh-in-motion purposes to reliably measure weights of individual vehicles crossing a bridge.
Considering the deficiencies described in the aforementioned systems and methods, it is therefore an object of this invention to provide a reliable and satisfactorily accurate system for determination of weight of vehicles moving across bridges. It is a further object that such a weighing system has reasonable equipment cost and reasonable installation cost for existing bridges, as well as for new bridge constructions. It is yet another object that Applicants' measuring system is by nature robust relative to effects of traffic and inclement weather, with key components of the system located in relatively protected locations, i.e. under a bridge, so that maintenance is infrequent, but in the infrequent event that maintenance is required no disruption of traffic on a bridge is required to perform such maintenance. Other objects of the invention will become apparent upon a reading of the following appended specification.